The high price and diminishing supply of natural gas and petroleum has caused the chemical and power industry to seek alternative feedstocks and fuels for the production of chemicals and the generation of electrical power. By contrast, coal and other carbonaceous fuels such as, for example, petroleum coke, petroleum wastes, biomass, and paper pulping wastes, are in great abundance, relatively inexpensive, and are logical materials for the art to investigate as alternative feedstock sources. Coal and other solid carbonaceous materials can be gasified, i.e., partially combusted with oxygen, to produce synthesis gas (also referred to hereinafter as “syngas”), which can be cleaned and used to produce a variety of chemicals or burned to generate power.
Gasification processes typically produce a crude synthesis gas with a molar ratio of H2 to CO of about 0.3:1 to 1.5:1, together with lesser amounts of CO2, H2S, water vapor, methane, and other materials. The molar ratio of H2 to CO in the product syngas is highly dependent on the feedstock and gasification process used therein, but generally falls within the above range. Different applications, however, require different H2 to CO molar ratios to utilize the syngas raw material efficiently. For example, Fischer-Tropsch and methanol reaction stoichiometries require a molar ratio of H2:CO of about 2:1, synthetic natural gas production requires about 3:1, acetic acid synthesis requires about 1:1, while the feedstocks for ammonia or hydrogen production require only the hydrogen component of syngas. This molar ratio can be adjusted by methods known in the art such as, for example, by the water gas shift reaction in which carbon monoxide is reacted with water to produce hydrogen and carbon dioxide.
The molar ratio of water to carbon monoxide of the syngas feed is an important parameter for proper operation of the water gas shift reaction section. A high H2O:CO molar ratio, typically about 1.5:1 to about 3:1, is advantageous to help control the temperature increase from the exothermic heat of reaction and to limit side reactions such as methanation. The H2O:CO molar ratio present in the syngas is dependent both on its method of production and on the operating parameters for that particular method. In addition, when syngas is used as a fuel for power plants, the presence of water in the syngas is sometimes desirable to retard fouling of the combustion turbine and other equipment.
For example, the amount of water present in a raw syngas produced by gasification is in part dependent on the feed method to the gasifier (e.g., water slurry or dry feed of the carbonaceous feedstock), the gasifier type, operating conditions, and the method used to cool the raw syngas from a gasification process. Often in water slurry-fed gasifiers, there is insufficient water in the cooled syngas effluent from the gasifier to operate a water gas shift reaction at the desired conversion. With dry coal feed gasifiers, the water content in the raw syngas is even lower than with a water slurry feed method. Thus, a method of humidifying syngas is needed that will provide an wide range of H2O:CO molar ratios for a water gas shift reaction.
In addition to adequate humidification, it is also desirable to recover thermal energy efficiently from the raw syngas while retaining the ability to control the amount of water present. Many methods have been proposed in the art for cooling raw synthesis gas, including full water quenching, diluent gas cooling, and radiant cooling. In a full water quench design such as, for example, as disclosed in U.S. Pat. No. 2,896,927, the hot raw syngas from the partial oxidation section is immediately contacted with a reservoir of flowing water without intermediate heat exchange. The raw syn gas is rapidly cooled by direct contact heat exchange and a fraction of the sensible heat content of the syngas heats and evaporates quench water. The quenched outlet syngas typically is saturated with water to its equilibrium level and has a H2O:CO molar ratio approximately in the range of about 1.5 to about 2.7:1 and an outlet gas temperature of around 185 to about 245° C., depending on system pressure. Although capable of providing a humidified syngas, the above full quench design is energy inefficient. Because of the high temperature of the raw syngas, the thermal energy of the raw syngas is degraded to a much lower temperature range and is incapable of generating valuable high pressure steam. Moreover, the ability to precisely adjust the H2O:CO molar ratio is severely limited.
In radiant cooling designs such as, for example, as described in U.S. Pat. No. 4,889,657 and in C. Higman and M. van der Burgt “Gasification” (Elsevier, 2003), Chapter 5, Section 5.3.5, the hot crude syngas leaves the partial oxidation section of an entrained-flow gasifier and enters a heat exchanger section that relies on a radiant heat transfer system to generate steam in tubes built into the heat transfer surface at the perimeter of a cylindrical gas flow area. Gas typically leaves the radiant cooler section at a temperature less than about 800° C. Radiant cooling processes, therefore, enhance the energy efficiency of the gasification process by generating steam from the sensible heat of the raw syngas, but do not address the humidification of the cooled syngas.
Cooling the raw syngas in a radiant cooling section followed by total quench cooling also has been described, for example in EPRI report AP-3486, “Cost and Performance for Commercial Applications of Texaco-Based Gasification-Combined-Cycle Plants”, Volume 1, Final Report Project 2029—10, Apr. 1984. Other combinations of radiant and quench cooling have been disclosed, for example, in U.S. Pat. Nos. 4,502,869 and 4,559,061, in which a fraction of the raw syngas is passed to a water quench section and a remaining fraction is passed to a radiant cooling section. Another example of a gas cooling design is described in C. Higman and M. van der Burgt “Gasification” (Elsevier, 2003), Chapter 5, Section 5.3.3. In this design the raw syngas leaves the partial oxidation section at a temperature of about 1200 to 1500° C. of an entrained-flow gasifier and is mixed with previously cooled, recycled gas at about 280° C. in sufficient quantity to cool the mixture to about 700 to 900° C. The mixed gas is then further cooled by generating steam in a convective syngas heat exchanger to about 280° C.
The various humidification and cooling methods disclosed in the art do not address the problem of producing a humidified syngas having a broad range of H2O:CO molar ratios that can be varied in response to multiple downstream syngas requirements while, at the same time, efficiently recovering the thermal energy of the syngas stream. Therefore, a simple, reliable, and energy efficent method for humidifying a syngas stream that is capable of producing a full range of H2O:CO molar ratios is needed. In addition, there is need for a process in which the H2O:CO molar ratio of a syngas stream can be precisely controlled and varied over time as required for one or more downstream applications.